1. Field of the Invention
The present invention relates to a rack and pinion type steering apparatus and, more particularly, to improvement of a rack guide thereof.
2. Description of the Prior Art
A rack and pinion type steering mechanism has been used for a steering apparatus of an automobile. In such a steering mechanism, a rack shaft is disposed with a rack guide (a support yoke) on the back of its position engaging with a pinion shaft so that the rack shaft is securely engaged with the pinion shaft. The rack guide is fitted into a guide bore which is provided in the direction perpendicular to the rack shaft in a rack housing, and is supported in the guide bore so that the rack shaft is properly pressed against the pinion shaft by pressure of a spring mounted in the guide bore. FIG. 1 is a plan view of a conventional rack guide and FIG. 2 is a side elevation thereof. The rack guide in the form of a cylinder has on its one end-surface 101 of the axial direction a concave surface engaging with a circumferential surface of the rack shaft as shown in FIGS. 1 and 2. The rack guide which has been made of metal is now superceded by that made of synthetic resin in order to obtain a light-weight apparatus.
However, when the rack guide of synthetic resin is cooled in forming process, the portion of the one end-surface 101 formed to be concave is likely to shrink inwardly as shown in FIG. 3. As a result, there exists a problem that the diameter of the one end-surface 101 becomes smaller than that of the other end-surface 102, consequently, an outer circumference 103 becomes wider as it goes from the one end-surface 101 toward the other end-surface 102, making a taper-form. It has been impossible to solve this problem by adjusting a metal mold. In order to solve this problem, an outer diameter of the whole rack guide, formed by the metal mold, is cooled so that it can be wider than that of its design and that the outer circumference 103 is ground so that the diameter of the one end-surface 101 is equal to that of the other end-surface 102. This method, however, leaves a disadvantage that it requires the grinding process in addition to the forming process. It also leaves another disadvantage that grinding of the outer circumference 103 is so difficult that it leads to lower efficiency of the work.